This invention relates to a highly efficient refrigeration system and process, employing two absorption refrigeration cycles in series operation, the first cycle comprising an ammonia-aqueous ammonia system and the second cycle comprising a novel absorption system and process, driven by low-grade heat and operated at pressures substantially below atmospheric, which provides cooling at temperatures as low as about -10.degree. C. Solutions of normally liquid, mutually soluble components, having substantially different boiling points, are employed as refrigerant and absorbent. Typically, the absorbent comprises a distillation bottoms fraction containing from about 65 to about 95 mol. % of the higher-boiling component and the refrigerant comprises a distillation overhead fraction containing from about 93 to about 99 mol. % of the lower-boiling component. Multi-effect modes may be employed. Heat pumped up in the first ammonia cycle is absorbed in the second cycle by refrigerant at the coolant temperature achieved therein.
The absorption cooling process enables thermal energy to be converted directly into a cooling effect and thus provides the basis for an attractive refrigeration process. In practice the absorption cycle has been employed with only a few absorbent-refrigerant combinations although many components have been suggested based on their respective physical properties and on theoretical consideration of the absorption cycle. In this cycle, refrigerant is first evaporated to afford a cooling effect, refrigerant vapor is then taken up in an absorbent, with evolution of heat, and finally the rich absorbent solution is subjected to fractionation to regenerate the refrigerant as an overhead stream for condensation and recycle to the evaporation step.
The absorption cooling process is usually operated at or near atmospheric pressure. An ideal refrigerant has been defined as one permitting boiling at about 5-10.degree. C. and condensation at about 38.degree. C. or higher. An ideal absorbent should be a liquid having a relatively high boiling point. A suitable refrigerant-absorbent combination should exhibit a significant negative deviation in vapor pressure from an ideal solution. Commercial utilization has been generally limited to two systems, one employing water as refrigerant with lithium bromide brine as absorbent, and the other employing ammonia as refrigerant with aqueous ammonia as the absorbent.
In theory, the efficiency of an absorption cycle is dependent only upon the temperature levels achieved in the evaporator, absorber, generator and condenser sections of the cycle. However, the permissible operating temperatures for these sections exhibit an interdependence which serves to limit the effective performance of the system. For example, the refrigerant partial pressure in the absorber will determine the operating temperature in the evaporator. Similarly, the refrigerant partial pressure in the generator will determine the temperature in the condenser. The operating temperatures in the evaporator and condenser are fixed by the temperatures and concentrations maintained in the absorber and generator.
Existing refrigerant-absorbent systems have been limited either by their physical properties or by the relatively low thermal efficiencies that are realized. For example, the lithium bromide-water system is subject to crystallization of the salt phase if temperatures are set too low; and in the evaporator section, at the lowest temperature in the cycle, icing may occur if this low temperature reaches as low as 0.degree. C. Ammonia-aqueous ammonia systems are often employed despite their generally low coefficients of performance; these systems have greater flexibility in the choice of operating conditions and are not subject to the possibilities of crystallization and icing. In the selection of absorption refrigeration as an alternative to electrically-driven or steam-turbine driven mechanical refrigeration, the choice has generally been limited by economic considerations involving the selective use of a particular form of energy rather than another.
The flexibility and relative lack of corrosivity and related physical problems in the ammonia-aqueous ammonia system make it an attractive focal point for improved production of refrigeration in an economically attractive manner.
Pertinent prior art includes Institute of Gas Technology Research Bulletin No. 14, entitled "The Absorption Cooling Process", which presents a comprehensive review of the literature up to 1957. Refrigerant-absorbent combinations are discussed thoroughly and evaluated in terms of practical and theoretical considerations. Such combinations include: ammonia-aqueous ammonia, water-aqueous lithium bromide, dichloromethane-dimethoxytetraethylene glycol.
In two articles, Hansworth, W. R., "Refrigerants and Absorbents", Part I, Refrig. Eng., 48, 97-100 (1944); Part II, ibid., 48, 201-205 (1944), there is presented an extensive review of the field and which focuses on the system water-diethylene triamine as a promising one for development in light of the properties set forth as desirable in each component. Hainsworth also presents a circular chart, attributed to Taylor, R. S., Refrig. Eng., 17, 135-143, 149 (1929), listing some 66 compounds, from carbon dioxide to glycerol, in order of ascending normal boiling points. This list includes both water and ethylene glycol. In an appendix table of refrigerant-absorbent combinations, ethylene glycol is listed frequently as an absorbent (with, for example, methyl alcohol, ethyl alcohol, n-propyl alcohol, ethylene diamine, n-amyl amine, morpholine, and N-methyl morpholine) and is one of some 27 components proposed as absorbent in combination with water as refrigerant.
In a contemporary publication, Taylor, R. S., "Heat Operated Absorption Units", Refrig. Eng., 49, 188-193, 207 (1945), presents a detailed survey of progress in the design of absorption refrigeration systems. Although water is frequently mentioned as a refrigerant, no mention is made of ethylene glycol.
Among prior patents, U.S. Pat. No. 1,734,278 discloses, as an improvement over the ammonia-water absorption system, the use of a methyl amine as refrigerant and an alcohol, such as glycerine, as the absorbent, particularly when having dissolved therein a metal salt of calcium, barium or lithium. U.S. Pat. No. 1,914,222 discloses ethylene glycol, alone or in mixture with water, as absorbent for use with methylamine as refrigerant. Hydrogen is present as an auxiliary gas. U.S. Pat. No. 1,953,329 discloses means for avoiding the freezing of the refrigerant by mixing with a minor quantity of the absorbent agent in the evaporator zone. U.S. Pat. No. 1,955,345 discusses problems with an ammonia-water system, such as the evaporation of water with ammonia and consequent loss of efficiency.
U.S. Pat. No. 1,961,297 discloses apparatus for use with a water-glycerol mixture at sub-atmospheric pressures. U.S. Pat. No. 2,308,665 discloses water or low-boiling alcohol as refrigerant, and a polyamine or polyamide as absorbent, and cites the methyl amine-ethylene glycol system. U.S. Pat. No. 2,963,875 discloses a heat pump system, employing liquids miscible at elevated temperatures, such as triethyl amine-water. Water and glycols are treated similarly as examples of high-boiling liquids.
U.S. Pat. No. 3,296,814 employs lithium salt solutions as absorbents, typically lithium bromide in ethylene glycol-water. U.S. Pat. No. 3,388,557 claims as an absorbent a solution of lithium iodide in ethylene glycol-water. U.S. Pat. No. 3,524,815 discloses water as refrigerant with an absorbent comprising lithium bromide and iodide, water, and ethylene glycol or glycerine. U.S. Pat. No. 3,643,555 claims specific proportions of the lithium salts.
U.S. Pat. No. 4,127,010 discloses a heat pump apparatus wherein the absorber liquor is preheated during passage to the evaporator by heat exchange with available internal streams to maximize the utilization of available heat. U.S. Pat. No. 4,193,268 discloses an evaporation device which permits a controlled evaporation rate in response to internal pressure differentials. The heat transfer medium may comprise water containing a minor amount of ethylene glycol. Preferred refrigerants include various chlorofluoromethanes and ammonia. Provision is made for injection of evaporator bottoms into a precooler otherwise containing refrigerant being passed to the absorber.
The current economic climate calls for more efficient and more complete use of the available energy resources. There is a genuine need for more efficient absorption refrigeration cycle components. There is a similar need for the economies inherent in a refrigeration system that can utilize waste heat as its driving force.